detonation / knock issue on 340

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shimsham1

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Hey all, trying to work out some issues on my 340. has bad detonation / igintion sounding knocking. i did not build or know anything much about the engine. Has a holley carb, mopar performance high rise intake. I've changed plugs, timing, fuel, etc. i noticed that when i spray carb cleaner at the base of the carb to manifold gasket it wants to stall so i'm guessing bad seal/ vacuum leak there. does anyone have any other ideas for me to check where its coming from. its from idle to like 3000 rpm then it goes away and the car runs and sounds like an animal. i did not change the plugs or cap. i got this car from someone that had it on jackstands for 19 years due to a bad trans , never starting it. i got the trans rebuilt and have just started driving and attempting to tune. any help would be greatly appreciated
 
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What year car? Points or electronic ignition? Where is timing set currently? Is there a vacuum advance? What gasoline are you using? If spraying at the base, are you sucking fluid through the throttle shafts which will give you a false reading?
 
What year car? Points or electronic ignition? Where is timing set currently? Is there a vacuum advance? What gasoline are you using? If spraying at the base, are you sucking fluid through the throttle shafts which will give you a false reading?
74 with a 72 340. electronic ignition, will throw a timing light on it tonight and get back to you on that but ive gone up and down. Sound gets worse when i advance it. there is a vacuume advance and it is operating. 91 octane fuel.
 
Good job on diagnosing it.
Change the gaskets now. Vac leak.
i was thinking that too, vacuume gauge reads good on manifold and ported. but since spraying the base of the carb and getting the reaction I did ill try the gasket next.
 
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If the carburetor sat for 19 years and was used it could do with a disassembly and good cleaning. Did you replace he fuel filter.

What the part number on the carb?
 
If the carburetor sat for 19 years and was used it could do with a disassembly and good cleaning. Did you replace he fuel filter.

What the part number on the carb?
yes on the fuel filter and the fuel has been clear. carb is 4776-4
 
One thing at a time. Replace the intake gasket because it's a known issue. Then you can check out the timing and such.
 
yes you need to confirm your timing at various rpm.Start at about 10 initial and no more than 34 at 3000 or 3300.Watch the rate as well.
If it still detonates try some 94 octane fuel
 
Hey all, trying to work out some issues on my 340. has bad detonation / igintion sounding knocking. i did not build or know anything much about the engine. Has a holley carb, mopar performance high rise intake. I've changed plugs, timing, fuel, etc. i noticed that when i spray carb cleaner at the base ( intake manifold ) gasket it wants to stall so i'm guessing bad seal there. does anyone have any other ideas for me to check where its coming from. its from idle to lie 3000 rpm then it goes away and the car runs and sounds like an animal. i did not change the plugs or cap. i got this car from someone that had it on jackstands for 19 years due to a bad trans , never starting it. i got the trans rebuilt and have just started driving and attempting to tune. any help would be greatly appreciated

With the engine being totally unknown to you, I would fix the carb gasket first since that one is an obvious issue.

I would test to see what the cranking cylinder pressure is. Too high and you will have to address it.

I would also turn the engine over to top dead center on #1 compression stroke and make certain the rotor button is pointed toward the lt front intake bolt to verify/ eliminate a timing or chain issue. Also need to check the harmonic balancer marks to ensure accuracy.

Make certain the advance weights in the distributor are free and lube them up.

Take the gas line loose coming out of the pump and fill a small glass jar to rule out rust in the tank causing a lean issue. I would also just go ahead and replace the filter. I am assuming you already pulled the tank and drained the old fuel.
Don't put more than 3 or 4 gallons back in in case the old scale inside turns loose with new gas as you don't want to have to deal with draining or dropping a full tank. Find some 93 octane if available in your area.


Once all that is done and any problems corrected then I would start trying to tune.
 
Very hard to diagnose an unknown quantity......
So concentrate on what is known which is: heat enemy is the of detonation.
Do everything possible to get the engine to run as cool as possible.
- coolant system
- use VA
- cold air induction
- block/reduce the exh xover in the intake
- richen mixture slightly
- etc
 
Please don't use something flammable to diagnose a vacuum leak. You're literally spraying gas on a fire. Think about it. Use water in a squirt bottle. MUCH safer.

When you get the vacuum leaks fixed, run a compression test on it and post those numbers. The vacuum leak repair may fix it. Running lean is a BIG issue that causes detonation.
 
Very hard to diagnose an unknown quantity......
So concentrate on what is known which is: heat enemy is the of detonation.
Do everything possible to get the engine to run as cool as possible.
- coolant system
- use VA
- cold air induction
- block/reduce the exh xover in the intake
- richen mixture slightly
- etc
engine runs extremely cool, wont go above 190 no matter what. I have not tried messing with the fuel mixture yet and will do that this weekend.
 
Please don't use something flammable to diagnose a vacuum leak. You're literally spraying gas on a fire. Think about it. Use water in a squirt bottle. MUCH safer.

When you get the vacuum leaks fixed, run a compression test on it and post those numbers. The vacuum leak repair may fix it. Running lean is a BIG issue that causes detonation.
I will try this over the weekend.
 
With the engine being totally unknown to you, I would fix the carb gasket first since that one is an obvious issue.

I would test to see what the cranking cylinder pressure is. Too high and you will have to address it.

I would also turn the engine over to top dead center on #1 compression stroke and make certain the rotor button is pointed toward the lt front intake bolt to verify/ eliminate a timing or chain issue. Also need to check the harmonic balancer marks to ensure accuracy.

Make certain the advance weights in the distributor are free and lube them up.

Take the gas line loose coming out of the pump and fill a small glass jar to rule out rust in the tank causing a lean issue. I would also just go ahead and replace the filter. I am assuming you already pulled the tank and drained the old fuel.
Don't put more than 3 or 4 gallons back in in case the old scale inside turns loose with new gas as you don't want to have to deal with draining or dropping a full tank. Find some 93 octane if available in your area.


Once all that is done and any problems corrected then I would start trying to tune.




I have not tested cylinder pressure yet, i will this weekend

I was also thinking it could be a stretched timing chain and will do what you recommend

i checked the vacuume advance but not the weights! thats a good one i need to do too

the tank is totally clean, filter was changed and fuel passing through is clear.
 
also noticed that the carb to manifold gasket is very thin, this is my first go around with a holley, ive always been a thermoquad guy. the carb to manifold gaskets were always thicker. i am asuming with the leak either the base of the carb or the manifold plane is not 100% flat. would a thicker gasket help solve that?
 
Look and see how fast the curve is in the distributor, some one might have put MP springs in there and it advances really fast and will detonate. Easy check with the timing light.
Or turn the rotor by hand if it moves initally without resistance it is MP springs, should have some tension and get harder as you turn it towards full advance, just an idea.
 
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DON"T bother doing another thing after backing off the timing;
until the compression test results are in; else you may just be chasing your tail.



Exactly. If you know your timing is safe and your fuel is fresh and of the right octane and it’s still pinging ALL testing from this point forward should be with the engine shut off. Detonation is a killer of parts. When engines set this long anything can happen and tearing at least the intake off is cheap insurance. Gaskets are cheap.
 
also noticed that the carb to manifold gasket is very thin, this is my first go around with a holley, ive always been a thermoquad guy. the carb to manifold gaskets were always thicker. i am asuming with the leak either the base of the carb or the manifold plane is not 100% flat. would a thicker gasket help solve that?
Start here you need to make sure the engine is not sucking extra air.
Spray around intake and carb. Find leak.
Then you can start checking carb(which is way to small imho), timing and ignition.
Also what are specs of engine?
Cam
Pistons
Compression ratio
Heads
 
My recommendation is don't change ANYTHING until you get vacuum leaks fixed FIRST. you really will be chasing your tail unless you do that first, IMO.

....and I highly recommend the thick mounting gasket. They are over 1/4" thick. They insulate well against heat and can make up for small imperfections in the mounting surfaces.
 
engine runs extremely cool, wont go above 190 no matter what. I have not tried messing with the fuel mixture yet and will do that this weekend.
Start with the easy stuff first as others have said.
When you pull the spark plugs to run your compression test take some closeup pic's of the plugs and post them here. They will tell you a lot - A/F ratio, timing etc
 
Start with the easy stuff first as others have said.
When you pull the spark plugs to run your compression test take some closeup pic's of the plugs and post them here. They will tell you a lot - A/F ratio, timing etc
I pulled the original plugs yesterday, ill take pics of them when I get home and run the compression testing. I'm hoping the vacuum leak at the carb base to manifold is the issue. getting a thicker gasket to try that.
 
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